The Collaborative International Dictionary of English v.0.48:
Mitigator \Mit"i*ga`tor\, n.
One who, or that which, mitigates.
[1913 Webster]
Moby Thesaurus II by Grady Ward, 1.0:
26 Moby Thesaurus words for "mitigator":
alleviative, alleviator, anodyne, assuager, balm, calmative,
cushion, dolorifuge, lenitive, moderator, modulator, mollifier,
pacificator, pacifier, palliative, peacemaker, restraining hand,
salve, sedative, shock absorber, soother, soothing syrup,
stabilizer, temperer, tranquilizer, wiser head